1. Field of the Invention
This invention relates to a method for the manufacture of phthalic anhydride. More particularly, this invention relates to a method for producing phthalic anhydride by subjecting naphthalene or ortho-xylene in a gaseous phase to catalytic oxidation with a molecular oxygen-containing gas.
2. Description of Prior Art
Phthalic anhydride, as widely known, is produced commercially by passing a mixed gas consisting of naphthalene or ortho-xylene and a molecular oxygen-containing gas at elevated temperatures through a shell-and tube reactor packed with a catalyst thereby causing catalytic oxidation of naphthalene or ortho-xylene. A typical catalyst usable for this method comprises a nonporous inactive carrier and a catalytically active substance comprising 1 to 15% be weight of vanadium pentoxide and 99 to 85% by weight of titanium dioxide deposited in a layer of a thickness of 0.02 to 2 mm on the carrier and used in an amount such that the vanadium pentoxide content falls in the range of 0.05 to 3% by weight based on the catalyst (U.S. Pat. No. 3,464.930). There has been proposed another method which has a phosphorus compound further included in the catalytically active substance deposited on the carrier in the aforementioned catalyst (U.S. Pat. No. 3,684,741). A typical catalyst to be used in this method comprises a nonporous inactive carrier and a catalytically active substance consisting of 1 to 40% by weight of vanadium pentoxide and 60 to 99% by weight of titanium dioxide and superposed in a thin layer on the aforementioned carrier, with the catalytically active substance further containing therein rubidium and/or cesium in the form of an oxide in an amount of 0.15 to 1.5% by weight based on the titanium dioxide and having a vanadium pentoxide content of 0.05 to 4% by weight based on the catalyst (German Offenlegungsschrift No. P 24 36 009).
Generally it is known that in a catalyst for the oxidation of naphthelene which has vanadium pentoxide and titanium dioxide as main catalytic components, an alkali metal compound added to the catalytic components prevents naphthalene from excessive oxidation and improved selectivity for phthalic anhydride or for phthalic anhydride and naphthoquinone. Since the alkali metal compound is added in a large amount, however, the catalytic activity is notably lowered and the yield is lowered. Thus, it has been customary to add this compound only in a small amount as in the methods of the aforementioned patents. Particularly in the case of rubidium, even when it is added in a small amount, it exhibits a notably high effect as compared with potassium and other alkali metals. Thus, it has been held not advantageous to add rubidium in a large amount.
Besides, the conventional methods such as described above have proved unsatisfactory in terms of repression of by-products, life of catalyst, yield of phthalic anhydride, etc. These problems become quite conspicuous when naphthalene is used as the raw material. These problems gain in seriousness in proportion as the concentration of naphthalene or ortho-xylene in the feed gas increases. Economically, however, it is desirable that the concentration of naphthalene or ortho-xylene should be so high as to exceed the lower explosive limit. In the meantime, the formation of by-products can be decreased by carrying out the oxidation at as high a temperature, at as low a gas feed volume (for as long a retention time), and in as low a concentration of naphthalene or ortho-xylene as possible. Under the conditions satisfying all these requirements, however, the productivity of phthalic anhydride is low.
With a view to eliminating the drawbacks mentioned above, there has been proposed a method for effecting the gaseous-phase oxidation of naphthalene or ortho-xylene by using on the up-stream side of the flow of a mixed gas of raw materials a first catalyst carrying thereon a catalytically active substance consisting of vanadium pentoxide and titanium dioxide and containing rubidium in an amount of 0.01 to 0.3% by weight based on the titanium dioxide and containing no phosphorus and on the downstream side of the flow a second catalyst carrying therein a catalytically active substance consisting of vanadium pentoxide and titanium dioxide and containing phosphorus in an amount of 0.02 0.8% by weight based on the titanium dioxide and containing no rubidium (German Offenlegungsschrift No. P 25 46 268). Even this method has a problem that the yield of phthalic anhydride is not sufficient as a whole. This problem is particularly conspicuous when naphthalene is used as the raw material.
It is, therefore, an object of this invention to provide an improved method for the manufacture of phthalic anhydride.
Another object of this invention is to provide a method for producing phthalic anhydride in high yield and with high productivity.